Terran is very active and could always use new nobles. Plus there is a ton of room for very quick ascension in the hierarchy because 1) it is a Republic with regular gov't position elections and 2) because there seems to be an election for lordship position every couple weeks for you to throw your name into.
Also, we're fighting a war.
He already has a character in Morek. Unless he is willing to move that character or pause him, you won't see Kellaine joining Terran.
Is Sirion not active anymore? That is very unlike of Sirion to go quiet. If you don't want to emigrate to another continent, why not join Dunnera and see Aiden and Davis trying to fight each other for the second time?
